1. Field of the Invention
The present invention relates to improvements in manpower-assisting power devices suitable for application with, for example, a bicycle equipped with an assisting electric motor.
2. Description of the Related Art
Manpower-assisting power devices combining manpower rotation and rotation driven by an electric motor or other external source by means of a differential gear mechanism are described in Japanese patent tokkai S55-31644 (1980-31644), Japanese utility model jikko H2-391 (1990-391), and Japanese patent tokkai H5-319354 (1993-319354). The mechanism described in Japanese utility model jikko H2-391 (1990-391), for example, combines manpower with the power of an electric motor to enable faster travel than when utilizing the electric motor alone.
A manpower-assisting power apparatus including a mechanism for detecting any applied manpower-generated torque, and applying electric motor torque according to this detected manpower has also been proposed in Japanese tokkai S56-76590 (1981-76590). FIG. 7 shows the configuration of the manpower-assisting power apparatus based on Japanese tokkai S56-76590 (1981-76590).
The manpower-assisting power apparatus shown in FIG. 7 has a differential gear mechanism comprising pedal drive bevel gear 82, detection bevel gear 71, and a pair of small bevel gears 83 engaging these gears 82 and 71. Manpower rotation applied to pedal drive bevel gear 82 by means of pedal axle sprocket 14, chain 80, and pedal drive sprocket 81 is transferred to output bevel gear 85 by the revolution of small bevel gears 83 supported in a free rotating manner on small bevel gear rotation axle 84, and then through bevel gear 86 to axle 87.
Rotation of pedal drive bevel gear 82 and the opposing detection bevel gear 71 is hindered by spring 72. Pedal drive bevel gear 82 and detection bevel gear 71 thus rotate an angle proportional to the reaction force received from small bevel gears 83, and the amount of rotation is detected by potentiometer 73.
Small gear 74 is fixed to the rotational axis of electric motor 20, and engages gear 88 fixed to axle 87. Electric motor 20 generates torque according to the amount of rotation detected by potentiometer 73. As a result, power complementary to the manpower and generated by electric motor 20 is thus added to axle 87.
However, conventional manpower-assisting power apparatus combining, by means of a differential gear mechanism, rotation produced by manpower and rotation produced by the power of an electric motor or other external source, as described hereinabove with reference to FIG. 7, only has the effect of a simple power combiner or transmission apparatus, and is not able to provide power assistance proportional to the experienced manpower. To further illustrate this shortcoming, consider an electric motor-equipped bicycle employing a conventional manpower-assisting apparatus. In this case, the rider must perform two separate actions, 1) turning the pedals with the feet; and 2) controlling operation of the electric motor with, e.g., an awkward lever provided on the handlebar in a substantially simultaneous manner. As a result, operation is complicated and power assistance is delivered with no relationship to the force pushing the pedals. Further, operating errors can occur easily with the lack of natural feedback to the rider, thus making safe operation difficult, if not impossible. In effect, the rider is distracted from the road and his changing environment by control of the power assisting motor. Moreover, it is legally difficult in many countries, including Japan, for conventional self-propelled capable bicycles (utilizing, e.g. an electric motor) to be considered merely a bicycle in which a driver's license or permit is not required.
A possible solution has been proposed by implementing a manpower torque driving mechanism responsive to human applied pressure to the pedals. This results in a power system which incrementally increases or decreases the torque of the electric motor according to the detected human originated torque being applied to the pedals. And, it offers a simple, intuitive human interface for controlling the motor with improved feedback, and so-equipped vehicles and other devices can be operated easily without requiring fine motive power control as in a motor vehicle. However, when the manpower torque detection precision is not sufficient and output linearity is poor, not only does the sense of control feel unnatural, but dangerously rapid acceleration unintended by the rider may occur. In addition, sufficient power assistance may not be achieved when it is necessary to avoid danger. Therefore, a high precision torque detection apparatus is required for this type of electric motor-equipped bicycle, but high precision torque detection apparatuses are heavy, cumbersome and expensive.
Furthermore, when the battery or other energy source is depleted and power assistance cannot be obtained because of the torque detection mechanism, the pedals become more difficult to turn. This is a significant problem making riding the bicycle difficult or impossible for the most common users of electric motor-equipped bicycles, including people with low physical strength and people who frequently travel on hilly roads.